1. Field
The present disclosure relates generally to analog and digital signals and, in particular, to analog-to-digital converters. Still more particularly, the present disclosure relates to a method and apparatus for converting an analog input into a digital output using a conversion device capable of multiple resolutions.
2. Background
An analog-to-digital converter (ADC) is a device that quantizes an analog signal. The analog signal may be a continuous input signal representing a measurable parameter such as voltage or current. Quantization is the process of converting a continuous range of values into a finite range of discrete values. In this manner, quantizing an analog signal means creating a series of discrete digital values that represent the analog signal. These discrete digital values may be represented in, for example, binary format.
The resolution of an analog-to-digital converter is determined by the number of discrete values that can be produced over a range of analog values. In some cases, the resolution of an analog-to-digital converter may be expressed in bits and referred to as bit resolution. An analog-to-digital converter with an N-bit resolution may be able to produce 2N discrete values.
In other cases, the resolution of the analog-to-digital converter may be defined based on the type of measurements received as input. For example, the resolution of an analog-to-digital converter that receives an analog voltage signal as input may be the overall voltage measurement range for the analog-to-digital converter divided by the number of discrete values possible. In particular, the voltage resolution of an analog-to-digital converter may be determined by the least significant bit (LSB) voltage, which is the minimum change in voltage needed to cause a change in the digital output. As the least significant bit voltage decreases, the resolution increases.
In some cases, one portion of a range of measurements may be of more interest than a rest of the range of measurements. For example, a resistance temperature sensor may contain a resistor that changes resistance as a temperature of the resistor changes. As the resistance changes, the voltage signal that is output from the resistance temperature sensor also changes.
However, the change in voltage based on a one degree change in temperature may be smaller at colder temperatures, such as the cryogenic range, as compared to hotter temperatures. Consequently, the discrete values generated by an analog-to-digital converter receiving the voltage signal from the resistance temperature sensor may have a higher degree of error with respect to the colder temperatures as compared to the hotter temperatures.
Some currently available analog-to-digital converters may be unable to improve this error without increasing the bit resolution of the analog-to-digital converter. Increasing the number of bits may increase the amount of digital data that needs to be processed, stored, or both more than desired. Therefore, it would be desirable to have a method and apparatus that take into account at least some of the issues discussed above, as well as other possible issues.